This invention relates generally to non-complimentary or single-ended noise reduction systems and more particularly concerns non-complimentary noise reduction systems incorporating dynamically controlled filtering and low level expansion.
Dynamic filtering has been known and used for years in applications to reduce background noise present in audio signals. The basic principals of dynamic filtering were developed in the 1940's and remain the basis for the concepts of virtually all dynamic filter designs in use today.
One such design, for example, employs a low pass filter controlled from a detection circuit incorporating simple peak detection of the input signal. This design suffers from a limited range of accurate operation of the controllable filter.
Other designs exist in which dynamic filtering and low level expansion are combined to produce an improved noise reduction system. But these designs face another problem. One of the most important criteria for single-ended noise reduction systems utilizing in combination broad-band, low-level expansion and dynamically controlled low pass filtering is audible transparency. The low level expansion portion of a single-ended noise reduction system is only operational when signal levels drop below a threshold point. Therefore, in most applications and especially with composite music, the expansion portion of the system primarily operates only when signal levels are extremely low or in fade-in and fade-out portions of the music. But the dynamically controlled filter performs its function at nearly all times and continuously alters the band width of the system so as to reduce perceived noise in the audio signal. Thus the filter must also be extremely transparent at all times so as to avoid any loss of desirable audio information, such as would be the case if the filter were to close at low signal levels and reduce the band width so as to produce a noticeable loss of high frequency information.
In most known systems employing a dynamically controlled low pass filter, the control signal is produced in a substantially similar manner. The input signal is first high pass filtered so as to remove low frequency signals which are to have no effect on the dynamically controlled low pass filter. Thus, the filter control circuit is responsive only to the mid and high frequency portion of the audio band. This band-limited signal is then peak-detected or rectified and filtered so as to provide a substantially dc signal to control the dynamically controlled low pass filter. This method of producing the control signal results in an extremely limited range of response to the amplitude of the input signal. If the system is set up so that low signal levels will allow the filter to operate, normal and high amplitude input signals would obviously cause the filter to open to the end limit of the spectrum, therefore having no effective noise reduction provided at these normal and higher amplitude signals. Conversely, if the system is set up so that normal and high amplitude signals produce the desired effective response of the low pass filter, then low signal levels will cause the filter to remain closed, thereby reducing the high frequency content of the audio signal and producing an audible loss of high frequency audio information.
This problem was resolved in my U.S. Pat. No. 4,647,876, issued Mar. 3, 1987, by compressing the signal before presenting it to the dynamic filter control circuit. In that system, a 2 to 1 compression ratio was utilized compressing the dynamic range in half so that the dynamic filter would operate over twice its normal range of input level. The compressor operated in conjunction with a mis-tracking device to reduce the compression ratio at low signal levels. Without the mis-tracking device, a signal having a high degree of background noise would be compressed in some cases so that the amplitude of the noise floor could be of a sufficient magnitude so as to hold the dynamically controlled low pass filter open and not provide the desired amount of noise reduction. It is therefore apparent that the threshold control of that system affects both the threshold of expansion in the low level expansion circuit and the low level sensitivity of the dynamically controlled filter.
While that system overcame most limitations over the prior art systems, it still suffered from several problems. First, the compression system response characteristics for its attack and release times had to be optimized for the desired response of the composite low level expander, so as to avoid any perceived pumping in the operation of the low level expansion system. This response characteristic may not have been the desired response for the implementation of the dynamic filter control system. Second, further improvements could be made in the dynamic range of the dynamically controlled filter but, in that system, an increased compression ratio would be required, again complicating the design because the compressor and expander circuits operate conjunctively. Furthermore, while altering the compression and expansion ratio of that system could improve its dynamic range, a sacrifice of the transparency of the system might result. Third, that system does not lend itself to easy implementation of miniaturization of the circuit, such as hybrid circuit technology and large scale integration or LSI applications.